It looks like all the legacy threads that were started in 2016 in the holiday forums have completely vanished. I just get the generic "Insufficient Holiday Cheer" page and redirect to the holiday forum.
eg (from browser history)https://forums.penny-arcade.com/discussion/208783/the-year-that-just-keeps-on-giving-news-containment-thread/p9https://forums.penny-arcade.com/discussion/208670/show-us-your-pets/p10https://forums.penny-arcade.com/discussion/208708/that-gosh-dang-holiday-moba-thread/p7https://forums.penny-arcade.com/discussion/208710/final-fantasy-record-keeper-and-brave-exvius-too-holiday-events-abound/p16
I assume this is intentional, but a bunch of them we had just picked right up where we left off, but with current events, so... not entirely certain? The forum index is still listing that there should be 10 pages of threads, but they stop at around page 5, so again, unclear.